This application claims priority from European Patent Application No. 03077155.4, filed Jul. 9, 2003, herein incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a mirror having a mirror surface, a lithographic apparatus, a device manufacturing method and a device manufactured thereby.
2. Description of the Related Art
In a lithographic apparatus the size of features that can be imaged onto the substrate is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to be able to image smaller features. While most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation, for example of around 13 nm. Such radiation is termed extreme ultraviolet (EUV) or soft x-ray, and possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings.
Some extreme ultraviolet sources, especially plasma sources, emit radiation over a wide range of frequencies, even including infrared (IR), visible (VIS), ultraviolet (UV) and deep ultraviolet. These undesired frequencies will propagate and cause heating problems in the illumination and projection systems and cause undesired exposure of the resist if not blocked. Although the multilayer mirrors of the illumination and projection systems are optimized for reflection of the desired wavelength, for example 13 nm, they are optically flat and have relatively high reflectivities at IR, visible and UV wavelengths. It is therefore desirable to select from the source a relatively narrow band of frequencies for the beam of radiation. Even where the source has a relatively narrow emission line, it is desirable to reject radiation out of that line, especially at longer wavelengths. It has been proposed to use a thin membrane as a filter to perform this function. However, such a film is very delicate and becomes very hot, 200-300° C. or more, leading to high thermal stresses and cracking, sublimation and oxidation in the high power levels used in a lithographic projection apparatus. A membrane filter also generally absorbs at least 50% of the desired radiation.
U.S. Pat. No. 6,678,037, incorporated herein by reference, describes a lithographic projection apparatus wherein a grating spectral filter is used in the radiation system of the lithographic projection apparatus. This grating spectral filter is designed for passing radiation of desired wavelengths to form a beam of radiation and for deflecting radiation of undesired wavelengths. The grating spectral filter is substantially formed of a material having a complex refractive index close to unity at the desired wavelengths and includes silicon protrusions. The protrusions have a laminar sawtooth profile or a laminar square wave profile, as shown in FIGS. 3 and 4 of U.S. Pat. No. 6,678,037, respectively, and are present on a mirror, having a mirror surface.
U.S. Patent Application Publication 2003/0058529 A1 discloses a grating structure on a mirror substrate etched to have a grating period causing diffracting, and a multi-layer coating deposited on the grating structure. Further, U.S. Pat. No. 6,469,827 discloses blazed gratings that include a series of ramps. In an embodiment, the blazed grating is constructed on a substrate before a reflective multilayer, for example alternating Si and Mo layers, is deposited over the grating. A disadvantage is that Si does not block IR radiation very well.